Crystalline titania having nanotube crystal shape and process for producing the same

ABSTRACT

Crystalline titania of which the crystal shape is a novel nanotube. This crystalline titania is produced by treating crystalline titania with an alkali. This crystalline titania is used as an ultraviolet absorber, a masking agent, an adsorbent and an optically active catalyst.

This application is a divisional application filed under 37 CFR §1.53(b)of parent application Ser. No. 08/937,885, filed Sep. 25, 1997, now U.S.Pat. No. 6,027,775.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Titania (TiO₂) is excellent in properties such as ultravioletabsorbability, adsorbability and the like. Accordingly, it has beenwidely used as a material in such applications as {circle around (1)} anultraviolet absorber, a masking agent in an anti-sunburn agent, a paint,a film and the like; {circle around (2)} an absorber, an adsorbent, adeodorizer and the like.

Further, nowadays the superior photocatalytic activity of titania isgiven attention. The titania is applied to environmental cleanup or thelike upon decomposition of carbonaceous gas or nitrogen oxides whileutilizing the superior properties thereof, such as oxidation orreduction.

The improvement of the properties of titania, especially thephotocatalytic activity in the above-mentioned usages has been indemand.

2. Description of Related Art

As one of the conventional technologies for improving the properties oftitania, it is known that if titania is doped with SiO₂, the specificsurface area can be increased.

In order to improve the photocatalytic activity, the present inventorstried to chemically treat the TiO₂ powder obtained by a sol-gel method,having large specific surface area, with an NaOH aqueous solution toimprove the photocatalytic activity, and reported this technology in thefollowing literature.

(1) “Preprints of Symposium of Catalyst Chemistry related to Light”,Jun. 6, 1996, held by Rikagaku Kenkyusyo and Catalyst Academy, p. 24-25

(2) “Preprints of Annual Meeting of The Ceramic Society of Japan 1996”,Apr. 2 to 4, 1996, p. 170

SUMMARY OF THE INVENTION

Taking aim at improving the catalytic activity as the properties ofcrystalline titania, further investigations were made. Meanwhile, it hasbeen found that where the crystalline titania is treated with an alkali,if certain conditions are met, a titania crystal in a nanotube form,which has hitherto been unknown, is formed, leading to theaccomplishment of the present invention.

It has been hitherto considered that the crystal shape of crystallinetitania has only a spherical shape or a needle shape, whether it is ananatase type or a rutile type, so long as the present inventors know.

The present invention is to provide crystalline titania of a nanotubewhich has a novel crystal shape. The diameter of the nanotube variesdepending on the production conditions and the like. It is approximatelybetween 5 and 80 nm in many cases. The crystal structure which is easyto obtain is an anatase type.

This nanotube is formed by treating crystalline titania with an alkali.In order to increase the yield, the alkali treatment can be conducted ata temperature of from 18 to 160° C. using from 13 to 65 percent by outerweight of sodium hydroxide.

Since the nanotube is a hollow crystal, the specific surface area isincreased as compared with a solid crystal such as a needle crystal, andthe specific surface area in the volume occupied is more increased.Accordingly, it is expected to markedly improve the properties of thecrystalline titania. Further, this crystal is expected to find novel usein filters and the like upon utilizing the nanotube shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a model shape of crystalline titania in thepresent invention.

FIG. 2 is a transmission electron micrograph of Sample No. 1-11(40%×110° C.×20 hrs) in Example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The crystalline titania of the present invention is a nanotube as shownin FIG. 1.

The diameter of this nanotube varies depending on the productionconditions. It is usually between approximately 5 and 80 nm. The lengththereof also varies depending on the production conditions. It isusually between 50 and 150 nm. The thickness thereof is usually between2 and 10 nm.

With respect to the crystal system of this nanotube, the anatase type iseasily obtainable as shown in Tables 1 and 2.

The process for producing crystalline titania in the present inventionis described below. In the following description, “% by weight” whichrefers to the alkali concentration means outer percent by weight.

The crystalline titania which has a nanotube shape in the presentinvention is produced by treating a titania powder with an alkali.

(1) Production of a Titania Powder

The titania powder (crystalline titania) used herein has a particlediameter of, usually from 2 to 100 nm, preferably from 2 to 30 nmwhether it is an anatase type or a rutile type.

Examples thereof include titania powders produced from a titanium oresuch as anatase, rutile, brookite and the like by the following knownliquid phase method, vapor deposition method or sol-gel method.

“Vapor deposition method” here referred to is a method in which titaniais produced by hydrolyzing a titanium ore with a heating strong acidsuch as sulfuric acid or the like, and heating the resulting hydroustitanium oxide at from 800 to 850° C.

“Liquid phase method” here referred to is a method in which titania isproduced by contacting TiCl₄ with O₂ and H₂.

“Sol-gel method” here referred to is a method in which titania isproduced by hydrolyzing titanium alkoxide including Ti(OR)₄ in analcohol aqueous solution to form a sol, adding a hydrolase to the sol,allowing the mixture to stand for gelation, and heating the gel.

(2) Alkali Treatment

In the alkali treatment, a titania powder is dipped in from 13 to 65% byweight of sodium hydroxide at a temperature of from 18 to 160° C. forfrom 1 to 50 hours. Preferably, it is dipped in from 18 to 55% by weightof sodium hydroxide at a temperature of from 18 to 120° C. Morepreferably, it is dipped in from 30 to 50% by weight of sodium hydroxideat a temperature of from 50 to 120° C. for from 2 to 20 hours. At thistime, when the alkali concentration is high, the temperature may be low(refer to Sample Nos. 1-9 and 2-4). When the temperature is high, thealkali concentration may be relatively low (refer to Sample Nos. 1-8 and2-3).

When the concentration of sodium hydroxide is less than 13% by weight,the reaction time is too long to form a tube, and it is not efficientfrom the industrial viewpoint. When it exceeds 65% by weight, the tubeis hardly formed (refer to Sample Nos. 1-15, 1-16, 1-17, 2-10, 2-11 and2-12). When the temperature is less than 18° C., the reaction time forforming a tube is prolonged. When the temperature exceeds 160° C. , thetube is hardly formed.

As supported in Examples to be described later, the nanotube crystalaggregate can hardly be produced without the above-mentioned ranges. Atthis time, the alkali treatment may be conducted in an open vessel, thatis, under normal pressure (at atmospheric pressure). It is, however,advisable to conduct it in a sealed vessel. The evaporation of water issuppressed in the sealed vessel to stabilize the alkali concentration.When the temperature is increased to 100° C. or more in the sealedvessel, the pressure is increased, and a nanotube having a smalldiameter is easily produced as compared with the alkali treatment in anopen vessel. When the alkali treatment is conducted in the sealed vesselunder increased pressure of 1.5 atm (calculated), a nanotube having asmall diameter of from 5 to 10 is obtained.

The alkali treatment includes a step of water-washing as a finalstep. Itis advisable to neutralize the resulting product with an inorganic acidsuch as dilute hydrochloric acid or the like.

(3) Heat Treatment

The above-obtained nanotube titania may further be heat-treated at from200 to 1,200° C. for from 10 to 400 minutes, preferably at from 300 to800° C. for from 60 to 160 minutes. This heat treatment is expected toimprove the crystallinity of TiO₂ and to increase the catalyticactivity. The nanotube does not collapse through this heat treatment.Further, it does not collapse either upon using a pulverizer.

(4) Use

The specific surface area of the above-obtained nanotube titania in thepresent invention is by far larger than that of the spherical or needlecrystal.

Consequently, when this titania is used as an ultraviolet absorber, amasking agent, an adsorbent or an optically active catalyst, an increasein the specific surface area can be expected, and especially thespecific surface area per unit volume can be greatly improved.

When this titania is used as a catalyst, it may be ordinarily supportedon a metal such as platinum, nickel, silver or the like.

This nanotube titania can be used in such applications as {circle around(1)} a filter; {circle around (2)} a material with a new performancewhich is obtained by inserting an organic, inorganic, or metal materialtherein; and {circle around (3)} a magnetic substance with new magneticproperties which is obtained by inserting a magnetic material therein.

EXAMPLES

The present invention is illustrated specifically by referring to thefollowing Example.

(1) Production of Starting Crystalline Titania:

In order to give a composition of the formula

xTiO₂.(1−x)SiO₂

wherein x is 1 or 0.8,

commercial tetraisobutoxytitanium and tetraethoxysilane were dissolvedin an ethanol solution to conduct hydrolysis. To the resulting sol wasadded dilute hydrochloric acid as a hydrolase, and the mixture wasallowed to stand for gelation.

The gel was heated with an electric oven at 600° C. for 2 hours. Then,the heated product was pulverized using an agate mortar to obtain a finepowder.

The following two types of starting crystal titania, 1) and 2), wereprepared by this sol-gel method.

1) TiO₂ ... average particle diameter: approximately 15 nm, specificsurface area: 50 m²/g 2) 0.8TiO₂ . 0.2SiO₂ ... average particlediameter: approximately 6 nm specific surface area: 10 m²/g

Further, the following commercial crystal titania A was used a startingcrystal.

3) Commercial Crystal Titania A

Anatase-type crystal titania (TiO₂) produced by reacting an ilmenite orewith sulfuric acid by a vapor deposition method.

average particle diameter: approximately 20 nm

specific surface area: 50 m²/g

(2) Conditions of Alkali Treatment

Each of the titania powders was treated with alkali under conditionsshown in Tables 1 and 2 (those other than Sample No. 1-12 and 2-7 weretreated in a sealed vessel). The treated powders were subsequentlyneutralized with 0.1N—HCl aqueous solution. In this manner, the testpowders were prepared.

Each of the test powders was dispersed in an ethanol aqueous solution. Adroplet of the dispersion was dropped on a test stand using a pipet, andobserved using a transmission electron microscope to estimate the shapeof the crystalline titania.

The results are shown in Tables 1 and 2. From the results in thesetables, it is clear that no nanotube crystalline titania is obtainedwhen the alkali concentration is too low or too high.

In Tables 1 and 2, each judgement is indicated as follows;

“x”: A Comparative Example not within the present invention

“Δ”: An Example which produces a nanotube partially

“◯”and “⊚”: An Example which produces a nanotube excellently

The judgements of “◯” or “⊚” depend on a specific surface area of eachsample. The judgements of them are not always proper when a product isrequired other properties.

In Tables 1 and 2, the terms have the following meanings.

“%”: outer percent by weight

“tube/particle”: Particles are incorporated in tubes

“particle/tube”: Tubes are incorporated in particles.

In the crystalline titania in Table 1, x in the SiO₂ component wasreduced to approximately 0.01 through the alkali treatment. As shown inTable 2, even when the starting crystalline titania was composed of 100%TiO₂,the nanotube titania crystal was obtained. It is thus clear thatthe precipitation of titania nanotube has nothing to do with theaddition of SiO₂.

TABLE 1 Composition: 0.8TiO₂ · 0.2SiO₂ Type and Sample Conditions ofalkali shape of crystal Specific surface No. treatment precipitated area(m²/g) Judgement 1-1 2.5% × 100° C. × 60 h anatase particle 230 x 1-25.0% × 100° C. × 60 h ↑ ↑ 230 x 1-3 10% × 100° C. × 20 h ↑ ↑ 230 x 1-415% × 60° C. × 20 h ↑ tube/ 250 Δ particle 1-5 15% × 150° C. × 5 h ↑ ↑250 Δ 1-6 20% × 20° C. × 20 h ↑ ↑ 250 Δ 1-7 20% × 60° C. × 20 h ↑ ↑ 250Δ 1-8 20% × 110° C. × 20 h ↑ tube 300 ∘ 1-9 40% × 20° C. × 20 h ↑ ↑ 320∘ 1-10 40% × 60° C. × 20 h ↑ ↑ 340 ⊚ 1-11 40% × 110° C. × 20 h ↑ ↑ 420 ⊚1-12 40% × 110° C. × 10 h ↑ ↑ 480 ⊚ (reflux) 1-13 60% × 60° C. × 20 h ↑particle/ 400 Δ tube 1-14 60% × 60° C. × 40 h ↑ ↑ 400 Δ 1-15 68% × 60°C. × 2 h — particle 320 x 1-16 68% × 60° C. × 30 h — ↑ 350 x 1-17 68% ×110° C. × 20 h — ↑ 400 x x:Comparative Example (no good) Δ:Example(fair) ∘:Example (good) ⊚:Example (excellent)

TABLE 2 Composition: TiO₂ Type and Sample Conditions of alkali shape ofcrystal Specific surface No. treatment precipitated area (m²/g)Judgement 2-1 20% × 20° C. × 20 h anatase tube/ 200 Δ particle 2-2 20% ×60° C. × 20 h ↑ ↑ 200 Δ 2-3 20% × 110° C. × 20 h ↑ tube 300 ∘ 2-4 40% ×20° C. × 20 h ↑ ↑ 200 ∘ 2-5 40% × 60° C. × 2 h ↑ ↑ 400 ⊚ 2-6 40% × 110°C. × 20 h ↑ ↑ 420 ⊚ 2-7 40% × 110° C. × 20 h ↑ ↑ 500 ⊚ (reflux) 2-8 60%× 60° C. × 20 h ↑ particle/ 400 Δ tube 2-9 60% × 60° C. × 40 h ↑ ↑ 400 Δ2-10 68% × 60° C. × 10 h — particle 300 x 2-11 68% × 60° C. × 20 h — ↑350 x 2-12 68% × 110° C. × 20 h — ↑ 400 x 2-13 40% × 110° C. × 20 hanatase tube 300 ∘  :Commercial product A x:Comparative Example (nogood) Δ:Example (fair) ∘:Example (good) ⊚:Example (excellent)

What is claimed is:
 1. A process for producing crystalline titaniahaving a nanotube crystal shape which comprises treating a crystallinetitania starting material with an alkali to form a crystalline titaniahaving a nanotube crystal shape, wherein the alkali treatment isconducted at a temperature of from 50 to 120° C. with from 30 to 50percent by weight of sodium hydroxide in water.
 2. The process of claim1, wherein said alkali treatment is conducted under increased pressurein a sealed vessel.
 3. The process of claim 1, wherein after a step ofwater washing in the alkali treatment, the product is furtherneutralized.
 4. The process of claim 1, wherein after said alkalitreatment, the product is further heat-treated at a temperature of from200 to 1,200° C. for from 10 to 400 minutes.